One type of integrated circuitry currently used in the semiconductor industry comprises memory circuitry where information is stored in the form of binary data. The circuitry can be either volatile or non-volatile. Volatile storing memory devices result in loss of data when power is interrupted. In contrast, non-volatile memory circuitry retains the stored data even when power is interrupted.
The operation of memory circuitry, and particularly that of programmable metallization cells, has been disclosed in the Kozicki et al. U.S. Pat. Nos. 5,761,115; 5,896,312; 5,914,893; and 6,084,796, the disclosures of which are incorporated by reference herein. Such a cell includes an insulating dielectric material disposed between opposing electrodes. A conductive material is doped into the dielectric material. The resistance of such material can be changed between highly insulative and highly conductive states. In its normal high resistive state and to perform a write operation, a voltage potential is applied across the opposing electrodes. The electrode having the positive voltage applied thereto functions as an anode, while the electrode held at a lower potential functions as a cathode. The conductively-doped dielectric material has the capability of undergoing a structural change at a certain applied voltage. With such voltage applied, a conductive dendrite or filament extends between the electrodes, effectively interconnecting the top and bottom electrodes.
The dendrite remains when the voltage potentials are removed. This way, the resistance of the conductively-doped dielectric material between electrodes could drop by several orders of magnitude. Such material can be returned to its highly resistive state by reversing the voltage potential between the anode and cathode, effectively disrupting the dendrite connection between the top and bottom electrodes. Again, the highly resistive state is maintained once the voltage potential is removed. This way, such a device can function, for example, as a programmable memory cell.
The preferred resistance-variable material received between the electrodes typically comprises a chalcogenide material having metal ions diffused therein. A specific example is germanium selenide (GexSe100-x) diffused with silver (Ag) ions. One method of diffusing the silver ions into the germanium selenide material is to initially evaporate the germanium selenide glass and then deposit a thin layer of silver upon the glass, for example by sputtering, physical vapor deposition, or other known technique in the art. The layer of silver is irradiated, preferably with electromagnetic energy at a wavelength less than 600 nanometers, so that the energy passes through the silver and to the silver/glass interface, to break a chalcogenide bond of the chalcogenide material. As a result, the glass is doped with silver. If, however, too much silver is doped into the chalcogenide material, the chalcogenide material changes from an amorphous state to a crystalline one and, consequently, the operation of the programmable memory cell is adversely affected.
When a chalcogenide glass is used in a memory device to insure that its properties do not change during various processing steps associated with fabrication of the memory device, the chalcogenide glass must have a glass transition temperature (Tg) which is about or higher than the fabrication and processing temperatures used in the subsequent steps of memory device fabrication. If the processing and/or packaging temperatures are higher than the glass transition temperature, the amorphous state of the chalcogenide material may change to a crystalline state or the glass stoichiometry may change or the mean coordination number of the glass may change and the operation of the memory cell affected. As such, the glass stoichiometry of the chalcogenide glass must be chosen so that the glass backbone (before and after metal doping) and/or metal-doped glass has a glass transition temperature which is about or higher than the processing temperatures subsequent to the glass deposition or subsequent to metal doping of the glass.
Accordingly, there is a need for a chalcogenide glass material that will remain in a glass forming region when doped with a metal such as silver and which allows maximization of subsequent possible processing temperatures, as well as a method of forming such a non-volatile memory element.